As we will show in the following section, depending on the value of \(B_D\)  the late time ionisation fraction \(f_\mathbf {e}\)  can vary over many orders of magnitude. Previous studies of the \(21\,\mathrm {cm}\)  absorption profile have shown that the most interesting parameter region corresponds to \(f_\mathbf {e} \sim 0.1\%\)  and \(10^{-5} \lesssim \epsilon \lesssim 10^{-4}\)  [1]}, [2]}. In this case we expect also the effective dark photon coupling \(\delta \)  to be greater than \(10^{-5}\)  (unless there is an accidental cancellation between \(\lambda \)  and \(\kappa \) ). Experimental bounds from dark photon searches then require \(m_\mathbf {A} \gtrsim 10\,\mathrm {MeV}\)  [3]}, [4]}. At the same time, the masses of the millicharged DM particles should be as small as possible in order to enhance the effect of DM-baryon interactions. The main objective of the remainder of this letter will therefore be to identify the allowed mass ranges for \(m_\mathbf {e}\)  and \(m_\mathbf {p}\) .
